High-intensity discharge lamps (HID lamps) are widely used in automotive headlamp applications, since they can provide an intensely bright light. To ensure traffic safety, characteristics of such lamps such as beam profile, colour temperature, lamp driver characteristics, lamp dimensions, etc., are specified in different countries by the appropriate regulations. For example, in Europe, the beam profile that is to be emitted by a headlamp, i.e. the shape of the low (passing) beam and the shape of the high (driving) beam, is regulated by ECE-R98, where ‘ECE’ stands for ‘Economic Commission for Europe’, while design-specific aspects of discharge light sources for use in such headlamps are regulated by ECE-R99. Often, the lamps specified in these regulations are simply referred to by their designation, e.g. ‘D3S’, ‘D4R’, etc.
An R-type lamp (e.g. a D4R lamp), for use in conjunction with a reflector in a headlamp arrangement, has opaque ‘stripes’ arranged on the outer vessel to block, reflect or absorb some of the light in order to obtain the desired beam shape, for example to prevent glare and to obtain the required cut-off. These stripes generally comprise a ‘vertical’ stripe, i.e. a stripe arranged around the circumference of the lamp near the lamp base, and ‘horizontal’ stripes arranged along the length of the lamp, which is mounted essentially horizontally in a reflector of a lighting assembly, as described in EP 0 708 978 B1. The horizontal stripes in such a prior art lamp are positioned relatively high up on the sides of the lamp in order to achieve a high brightness level below the cut-off and a very low brightness level beyond the cut-off. At the same time, these effectively block a fraction of the light, which is effectively wasted. Therefore, the overall light output and efficiency for a lamp with such stripes is noticeably lower than for a comparable lamp without stripes. This loss of light is a considerable drawback, since an automotive lamp should deliver as much light as possible into the front beam for visibility and safety reasons. The light absorbed or blocked by the stripes also contributes to an over-heating of the lamp and can result in a shortening of the lifetime of the lamp. The reason for this is that the inner vessel or burner is relatively large, for example in the case of a 35 W D4R lamp, and there is only a small clearance between the burner and the outer vessel. The glass wall of the burner is therefore very close to the glass wall of the outer vessel, and the associated coefficient of thermal conductivity is high. The high temperatures cause an increase in the lamp voltage and a reduction in lumen output as the lamp ages, and can also lead to the development of flicker. The temperature increase is also associated with an unfavourable alteration in the colour of the light output by the lamp. Another unwanted side effect of the high temperatures is the development of cracks in the pinch region of the lamp under the vertical stripe, which can shorten the useful lifetime of the lamp.
Therefore, it is an object of the invention to prolong the lifetime of such a lamp.